Monitoring device for television systems



Dec. 18, 1945. T. T. GOLDSMITH, JR 2,391,090

MONITORING DEVICE FOR TELEVISION SYSTEMS Original Filed June 21, 1941HTTURNEY tube f uomoamanavrosroa I Thomas '1'. commits; In, Oedar em,hi. 3-. lnortodllennnumonthbmtcrinlna,

,NJ-i a motion application June 21, 1941, Serial No.

Divided and this application l lll aisiasuuna'mm;

- z cam. (01. its- 1.5)

This invention relatesto a device for monitor- 7 ing received televisionsignals. In carrying out the invention a cathode-ray tube is providedfor receiving a television picture signal and a vacuum tube is connectedin parallel therewith so thats composite mixture of the incoming picturesignal and a local blanking signal can be monitored on an oscillograph.

This is a divislon of my application Serial No. 399,055,:iiled June 21,1941, now Patent No. 2,373,114, granted April 10, 1945. I

The invention may be understood from the description in connection withthe accompanying drawing, which shows how various units may be connectedand is in part a diagram of connections ticed'.

In the drawing, reference character I indicates a television receiverfor an input signal through connection 2 either to a transmission lineor through the air. The receiver delivers a composite television signal3 that is delivered to a one-stage amplifier 4 which inverts the picturesignal. This inverted signal is passed through a direct current inserterI, to be described in detail later, which controls the reference levelfrom which the modulation occurs and yields a wave form which isopposite in polarity to the conventional wave form 3. This latter waveform is delivered by means of connection I to the grid 48 of the brightcathode-ray tube 8, thus providing the grid modulation to this tube 8for producing a bright reversed polarity television picture on thescreen 01 this tube.

The television receiver I delivers a horizontal sawtooth wave form bymeans of connection I I to a horizontal deflection I and to thehorizontal blanking generator I2. The receiver I also delivers avertical sawtooth wave form by means of connection It to the tube 8, andto the vertical blanking generator l4. The horizontal blanking generatorI2 and the verticalblanking generator I4 may consist, for example, ofcircuits oi the type described in the Campbell Patent 2,207,048.

The square wave form blanking pulses from generators I2 and I4 aresuitably mixed in the blanking mixer I1 and after passa e through adirect current inserter 25 the blanking signals are applied by means ofconnection 26 to the cathode 41 of the cathode-ray tube 8.

As described in the parent application, the

composite television picture signal in negative or reversed polarity isapplied to lead 40 and is illustrating how the invention may be pracofDelaware August passed through thecondenserfl to a diode tube 42 whichis' so connected as to provide direct plate of cathode-ray currentinsertion tocontrol the pedestal level of the television picture signalatfa fixed voltage relation for the grid 48 of the cathode-ray tube 8.The resistor 44 between tube 42 and lead .I is

of low resistance value, yet sumciently high in resistance so that thesynchronizing peaks on the composite wave form are practicallyignored;

consequently the signal appears on the plate 43 1 of the diode 42 asthough the pedestal level were essentially flat from the beginning tothe end of the blanking interval. A high resistance leak 45 is placedacross the diode 42 so that the voltages may return to normal aftermomentary unusualsurges. The condenser 4I tends to apply signals to theplate 43 which go positive and negative with respect to a predeterminedvoltage.

However, upon an attempt to drive the plate 43 positive with respect tothe cathode 4B, the tube 42 will conduct current and establish a biasvoltage on the condenser 4| so that no subsequent signals can go morepositive than the cathode 46. The reference voltage level is establishedin this way, and the television signals are maintained always negative,except for the synchronizing peaks, and the pedestal level is caused toassume a fixed voltage to maintain the proper brilliance setting of thegrid 48 in the cathode-ray tube 8'. In this way the receiverautomatically varies the absolute brilliance of the received picturewhen the transmitter sends one line, for example, which is essentiallybright on a standard picture of positive polarity and a subsequent linewhich is essentially dark in a standard picture of positiv polarity.'

It is to be remembered that the cathode-ray tube 8' reproduces thetelevision picture in negative polarity and therefore this directcurrent inserting diode 4| accomplishes the result of maintaining adirect current level formaximum white with means for modulating alwaystoward black from this level, whereas the conventional tube with apicture of positive polarity has a' It is'employed to prevent blankingslim-1 voltage across the potentiometer II. The high resistance I8shunts the diode and enables the circuit to return to its operatingrange automatically after an abnormal surge may have appeared. By meansof the potentiometer II the absolute value of the direct current voltagemay be set independent of incoming signals and this potentiometer servesas the brightness control of the cathode-ray tube 8. The vacuum tube IIis connected as a cathode iollower stage to pass the blanking signalswithout inversion of polarity but with reduction of source impedance.The blanking signals are applied from the direct current inserting diodeis to the grid 58 of this tube. The cathode 80 is connected directly tothe cathode 41 of the cathode-ray tube 8. The resistance II in thecathode circuit of tube 58 is of low value so as to transmit theblanking wave forms without distortion and so that changes in current inthe cathode-ray tube 8 will not cause an appreciable change in thedirect current level control circuit.

A vacuum tube In is connected in-parallel with the cathode-ray tube 8for the purpose of proirom app viding monitoring of thecombinationsignal resulting from the two signals applied, respectively, tothecathode 41 and the grid 48 of the cathoderay tube 8. The combinationsignal appears on the cathode-ray tube screen of tube 8 as changes inbrilliance, but for adequate control of these signals it is desirable toproduce and monitor the resulting wave form of the combination.Therefore the grid 1| of vacuum tube III is connected directly to thegrid 48 of the cathode-ray tube 8. The cathode 12 of tube I is connecteddirectly to the cathode ll of tube I. The plate I: of tube It thuscarries a current which is characteristic of both of the signals thatare applied to tube 8. A voltage develops across the plate load resistor18 which represents this combined signal. It may be applied by means ofa selector switch 18 to the vertical deflection system Vor y-axisterminals of the oscillograph ll while a sawtooth wave of properfrequency is applied to the H or :c-axis terminals in the usual way sothat the wave form will appear on the screen 18 of the oscillograph formonitoring purposes. The selector switch I. also provides for monitoringthe signal through the connection that appears upon the cathodes 41 and12 of tubes 8 and 1c and for monitoring the signal through theconnection 8| that appears upon the grids. 48 and II or these tubes,thus providing monitoring of any one oi. these signals independently.The switch 18 may also be used to monitor other points in the circuit iidesired by use of the lead 82.

What is claimed is:

1. In a television system, a cathode-ray tube and a vacuum tube, meansto apply a television picture signal tothe control grids of each of saidtubes. means to apply blanking signals to the cathodes of said tubes,and means for monitoring a composite mixture of said signals.

2. In a television system, a cathode-ray tube and a vacuum tube, meansto apply a television picture signal to the control grids of each ofsaid tubes, means to apply blanking signals to the cathodes of saidtubes, and an oscillograph for monitoring a composite mixture of saidsignals.

THOMAS T. GOLDSMITH, JR.

